In a photo-stimulable phosphor imaging system, as described in U.S. Pat. No. Re. 31,847 reissued Mar. 12, 1985 to Luckey, a photo-stimulable phosphor sheet is exposed to an imagewise pattern of short wavelength radiation, such as x-radiation, to record a latent image pattern in the photo-stimulable phosphor sheet. The latent image is read out by stimulating the phosphor with a relatively long wavelength stimulating radiation, such as red or infrared light. Upon stimulation, the photo-stimulable phosphor releases emitted radiation of an intermediate wavelength, such as blue or violet light, in proportion to the quantity of short wavelength radiation that was received. To produce a signal useful in electronic image processing, the photo-stimulable phosphor sheet is scanned in a raster pattern by a beam of light produced for example by a laser deflected by an oscillating or rotating scanning mirror, and the emitted radiation is sensed by a photodetector such as a photomultiplier tube to produce the electronic image signal.
In one type of scanning apparatus, the photo-stimulable phosphor sheet is placed on a translation stage, and is translated in a page scan direction past a laser beam that is repeatedly deflected in a line scan direction to form the scanning raster.
To optimize the signal-to-noise ratio (S/N) of the imaging system, it is desirable to collect as much of the emitted light as possible and to direct it to the photodetector. While the apparatus employed to collect the light may take various forms, one form of light collector is proposed in U.S. Pat. No. 4,346,295, issued Aug. 24, 1982, to Tanaka et al. The light collector proposed by Tanaka et al comprises a sheet of light transmitting material that is flat on one end, and rolled into an annular shape on the opposite end. The flat end of the light collector is positioned adjacent to the scan line on the photo-stimulable phosphor sheet. The light receiving face of a photomultiplier tube is placed against the annular end of the light collector.
Light emitted from the phosphor sheet enters the flat end of the light collector and is light piped to the photomultiplier tube. Improved light collection efficiencies are achieved by having two such light collectors one on each side of the scan line, or by placing a long narrow reflector opposite the flat end of the light collector to increase the collection window of the light collector. The transparent light collector has the drawback that it is inherently complicated to manufacture. Furthermore, the collection efficiency of transparent light guides is limited due to their absorption in the wavelength range of light emitted by the photo-stimulable phosphors (e.g. blue-violet).
In an attempt to provide an easily manufacturable, high collection efficiency light collector, a cylindrical integrating cavity light collector was constructed and tested. The cylindrical integrating cavity light collector, as shown in FIG. 3, comprises a hollow cylindrical light collection enclosure 10 having a pair of parallel slits 12 and 14 arranged opposite from one another along the axis of the cylinder. The inside of the cylinder was painted with a white diffusely reflective paint, and the light receiving surfaces of a pair of photomultiplier tubes 16 and 18 were positioned at each end of the cylinder. The light collector was positioned near the photo-stimulable phosphor sheet 20 and a laser beam 22 produced by a laser 24 was directed through the two parallel slits 12 and 14 to stimulate the phosphor sheet 20. The beam was scanned in a line scan direction by rotating a polygon mirror 26, and the photo-stimulable phosphor was scanned in a page scan direction by transporting the phospshor sheet 20 in the direction of arrow A by a translation stage not shown.
Light that was emitted from the phosphor upon stimulation, entered the collector from the bottom slit 14, and after a number of diffuse reflections from the inside wall of the cylinder, reached one of the photomultiplier tubes at the end of the cylinder. Filters 17 and 19 were placed over the faces of the photomultiplier tubes to absorb any of the stimulating radiation from the laser before reaching the photomultiplier tubes.
Experimental studies conducted with the cylindrical integrating light collector having diffusely reflective internal surfaces, identified a further factor that has an effect on the signal-to-noise ratio achievable with the photo-stimulable phosphor imaging apparatus. FIG. 4 shows a cross-section of the cylindrical integrating light collector useful in describing this factor. It was discovered that as the photo-stimulable phosphor sheet is scanned by the laser beam 22, a high percentage (up to 90%) of the stimulating radiation from the laser beam is reflected from the surface of photo-stimulable phosphor 20. If this reflected stimulating radiation is subsequently reflected to the surface of the photo-stimulable phosphor in a region outside the immediate scanning location, an untimely and undesirable stimulation of the phosphor takes place. The reflected stimulating radiation which is reflected back to the phosphor is called "flare."
This undesirable stimulation of the phosphor can occur within the collection window of the light collector as illustrated in FIG. 4A, or outside of the collection window of the light collector as illustrated in FIG. 4B. In the first case, illustrated by FIG. 4A the flare induced emission of light that is collected by the collector will be referred to as prestimulation. The prestimulation light is directed to the photomultiplier tubes and produces an additional component to the image signal. This signal component causes degradations in the image including a reduction in the contrast of images by prestimulation of high exposure areas and by the addition of unwanted signal to low exposure areas. Furthermore, "shadowy" types of artifacts are produced when an image in the form of a high exposure object on a low exposure background field is scanned. The signal-to-noise ratio in all image areas is degraded by flare induced emission, especially in regions of low x-ray exposure which are surrounded by high exposure regions. Also, the effect of laser noise is enhanced since a large area of the phosphor is exposed to a low level of stimulating intensity. The luminescence from this area will follow the fluctuations in laser power.
Where the undesirable flare induced emission occurs outside the collection window, upstream of the scan line, it is called predischarge. Predischarge occurs where reflected stimulating radiation is re-reflected from the underside of the light collector back onto the surface of the phosphor sheet 20 in the region upstream of the collection window of the light collector as illustrated in FIG. 4B. Although the emitted light produced by predischarge is not collected, and therefore does not directly appear in the image signal produced by the photodetector, the effect of predischarge reduces the signal level of the image and therefore lowers the overall signal-to-noise ratio achievable by the apparatus.
In an effort to improve collection efficiency and decrease prestimulation by reducing the number of internal reflections occurring inside the light collector, a V-mirror box light collector was designed having specularly reflective interior walls, and being tapered from the center toward the photomultiplier tubes located at both ends of the box. This V-mirror box collector is shown in FIG. 5. The V-mirror box collector is shown in FIG. 5. The V-mirror box collector was essentially a rectangular mirror box 10 tapered from a rectangular cross-section at the center to a square cross-section at each end, by sloping the top mirrors 28 toward the center. The bottom mirror 30 is flat and parallel with the surface of phosphor sheet 20.
In a furher refinement of the V-mirror box collector, shown in FIG. 6, the V-mirror box was tapered in two dimensions to further reduce the number of internal reflections, to thereby further improve the collection efficiency of the collector and further reduce prestimulation. The double V-mirror box light collector was essentially a rectangular mirror box tapered from a small rectangular cross-section in the center to a larger square cross-section at the ends by sloping the top mirrors 28 and the side mirrors 32 toward the center. The bottom mirror 30 remains flat and parallel with the surface of phosphor sheet 20.
To achieve optimum light collection, the bottom slit 14 must be as close to the light emitting surface of the photo-stimulable phosphor sheet 20 as possible and the bottom mirror 30 of the V-mirror box light collector must be as thin as possible. Furthermore, to reduce predischarge, the bottom of the V-mirror box light collector may be coated with a nonreflective coating so that stimulating light escaping beneath the bottom slot is not reflected back on to the surface of the phosphor sheet. Matte black finishes have the disadvantage that their reflective properties are drastically changed by contact and abrasion with other surfaces, which is likely to occur due to the extremely close position tolerance of the light collector to the surface of the phosphor. The specular reflectivity of surfaces such as smooth black anodized surfaces, is too high to be desirable.
These requirements make the double V-box light collector difficult to manufacture and use, although it provides a substantial improvement in light collection efficiency and reduction in prestimulation over light collectors of the light guide type.
It is the object of the invention therefore to provide improvements in light collectors for photo-stimulable phosphor imaging apparatus having high light collection efficiency, ease of manufacture, and low prestimulation and predischarge.